For the most time, I've been firmly in the largest camp when it comes to the Mono debate - the 'I don't care'-camp. With patent lawsuits being hotter than Lady Gaga right now, that changed. For good reason, so it seems; while firmly in the 'ZOMG-MICROSOFT-IS-T3H-EVILL!1!!ONE!'-camp, The-Source.com investigated the five most popular Mono applications, and the conclusion is clear: all of them implement a lot of namespaces which are not covered by Microsoft's community promise thing.

Have you heard of this C++ thing? Almost all the software you run is written in it

No, actually very little software on a typical Linux desktop is written in C++. KDE is the largest user, and most distros default to GNOME.

and it's neither encumbered nor irrelevant.

Yes, but I think it fails the "modern" requirement. C++ is better than C, but there are so many thing it got wrong that it's not even worth discussing (although there are a lot of things that I like very much about C++). There's also the fact that a lot of people are going to want a pointer-less language with fast, precise garbage collection. You can argue all you want, but it can hardly be argued that a good, open-source, unencumbered, fast, modern, GC-ed, OO language would not increase development for Linux.